> I see no way of migrating to 23.0 profile because of not-recompilable
> packages that are installed (over 4 years) which block --emptytree,
> and do not wish to be forced to migrate to merged-usr on an openrc box
> without a compelling need (on principle).

That sounds a bit like self-inflicted pain.

> Will patching back the 17.0 profile files into the portage tree if and
> when they are removed work?

Unknown. 

> Are there any options at all for this situation (like freezing the the
> last supported tree protecting it from emerge-syncs, and using an
> overlay for further updates?)

You can try to just skip these packages (with --exclude) during the
"emerge --emptytree ..." step. It should work, but no guarantees given.

-- 
Andreas K. Hüttel
dilfridge@gentoo.org
Gentoo Linux developer
(council, toolchain, base-system, perl, libreoffice)